This research is aimed at elucidation of the mechanisms by which growth hormone and prolactin are stored in and secreted from the anterior pituitary. To this end, the biochemical composition and enzymatic features of isolated pituitary secretory granules are under study, together with relevant properties of pituitary plasma membranes, cytosol, and mitochondria. High molecular weight disulfide-bonded polymers of growth hormone and prolactin have been identified within the secretory granules, and a novel thioltransferase purified from cytosol. Anion-sensitive ATPases have been found both in plasma membranes and in secretory granule membranes, and have proven to be inhibited by anion transport-blocking drugs. An SH-protease with a neutral pH optimum has also been identified in granule membranes. Further characterization of these newly identified activites, and identification of the granule membrane component involved in the binding of secretory granules to microtubules, should provide insight into the role of these proteins in the formation of secretory granules, the forms stored hormone, the intracellular transport of secretory granules, and the biochemical events which underlie pituitary secretory mechanisms.